The phase center of an antenna is a reference point from which electromagnetic radiation appears to emanate, all radiated fields measured on a spherical surface whose center coincides with the antenna phase center have the same phase. When navigation satellites report their position to the user, they actually report the location of the phase center of the antenna of their navigation transponder. The accuracy with which a user position is determined may be improved by more precisely identifying the coordinates of this phase center. For example, in instances in which the phase center of the antenna of the navigation transponder onboard a Global Positioning System (GPS) satellite is accurately measured and reported to the users, the user range error (URE) may be reduced, thereby making subsequent position determinations more accurate since the URE is the key performance parameter (KPP) of the entire GPS system.
In order to determine the location of the phase center of an antenna, the location of the phase center is generally measured at an antenna range test prior to deployment of the antenna. The information regarding the location of the phase center that is garnered during an antenna range test may then form the starting point for measurements of the displacement of the phase center of the antenna following its deployment; knowing the initial location of the phase center and measuring its displacement, the new instantaneous location is easily determined. With respect to antennas to be carried by satellites, the location of the phase center of the antenna may be measured with respect to the center of gravity of the satellite, such as a GPS satellite, during antenna range tests on the ground prior to launch. The information regarding the location of the phase center of the antenna is then employed as a starting point in order to measure the instantaneous location of the phase center of the antenna onboard the satellite following launch and during orbit. These measurements generally utilize monitor stations positioned around the world. Once the location of the phase center of an antenna has been determined, it may then be reported to the recipients of signals from the antenna, such as the users of a GPS satellite. By having an accurate estimate of the location of the phase center of the antenna, the URE may be reduced such that user positions can be determined with more accuracy. However, since the satellites carrying the antennas may orbit at a relatively large distance from the center of the earth, such as a distance of about 26,000 kilometers in one instance, measurements of the location of the phase center of an antenna may have errors that are larger than desired, such as errors that may exceed 18 inches (45.7 centimeters) in some instances. In this regard, these measurements of the location of the phase center of an antenna rely upon transmission of the radio frequency (RF) signals through the earth's atmosphere. Since the earth's atmosphere is a thermally unstable dispersive media causing variable delays to RF signals, the electromagnetic energy emitted by the antenna and, in turn, the RF signals received by the monitor stations on the ground may be altered by the atmosphere, thereby resulting in inaccuracies in the determination of the location of the phase center. As such, the information regarding the location of the phase center of the antenna that is provided to the recipients of signals from the antenna may differ from the actual location of the phase center of the antenna measured using the monitor stations. Moreover, the in orbit measurement process may take more time than is desired.
Additionally, the location of the phase center of an antenna will generally vary with changes in temperature and pressure due to, for example, thermal deformations of reflectors, feeds and/or antenna elements. As such, even if the location of the phase center of an antenna is accurately estimated at one point, this location may change as the pressure and temperature changes such that the location of the phase center of the antenna must be repeatedly determined.
In order to provide improved performance for at least some antenna-based systems, such as improved location determination for navigation systems, it may be desirable to provide an improved technique for determining location displacement of the phase center of an antenna. By more accurately determining the location of the phase center of an antenna, the recipient of signals from the antenna (the user) could also receive more accurate information regarding the location of the phase center of the antenna in order to reduce the errors that may otherwise exist in the determination of the user position.